Oocyte maturation of the swordtail (Kiphophorus hellerii) was investigated by light and electron microscopy. In the ovary of the swordtail, various staged oocytes were observed, Mature oocytes were located in ovarian cortex, meanwhile immature ones were positioned in ovarian medulla. The oocyte was surrounded by several structures or cells such as chorion, follicle cells, follicular theaca and ovarian epithelium, respectively, from the inside toward outside. Growing and maturing oocytes healed numerous microvilli which interconnected the oocyte and the follicle cells to communicate each other. The mature oocyte had the electron dense chorion which appeared to be ultrastructure of two layers and contained pore canals. Oocyte maturation was characterized by not only the enlarged cell size and well differentiated cell organelles, brit also the increases of fat droplets, pinocytotic vesicles and yolk granules.